XtRegisterDrawable associates the specified drawable with
the specified widget so that future calls to XtWindowToWidget with the
drawable will return the widget. The default event dispatcher will dispatch
future core events that arrive with the drawable to the widget as though
the event contained the widget's window, but the event itself will not be
changed in any way when being passed to even handler or action procedures.

If the drawable is already registered with another widget, or if the drawable
is the window of a widget in the client's widget tree, the results of calling
XtRegisterDrawable are undefined.

XtUnregisterDrawable removes an association
created with XtRegisterDrawable. If the drawable is the window of a widget
in the client's widget tree the results of calling XtUnregisterDrawable
are undefined.